An interface, esp. when returned from a method, is the best way the define a limited contract. If you return the concrete class, you have the following potential issues:

- Very broad, unspecific contract that may even obscure the methods purpose

- You cannot modify the contract without modifying the class AND vice versa

- Shrinking a contract (taking away elements) is far harder and more likely to cause breakages in other code than growing a contract

- Mocks become more cumbersome because the contract is so broad

- Changes to the concrete class cause ripple effects in code that doesn't care about the change